1. Field of the Invention
The invention relates to a heating resistance and heater suitable to, for example, for heating semiconductors, whose temperature distribution can be easily controlled.
2. Related Art Statement
In semiconductor producing systems, ceramic heaters have been applied for heating a wafer so as to deposit a semiconductor thin film on the wafer from gaseous raw materials such as silane gas by means of thermal CVD or the like. In such ceramic heaters, it is required to make the temperature of the heating face and the semiconductor wafer mounted thereon uniform at a high precision.
Several techniques for reducing the temperature distribution on the heating (mounting) face of the ceramic heater are known. For example, a so-called two-zone heater is such a heater. Such a two-zone heater has a ceramic substrate and inner and outer resistance heat generators embedded in the substrate. Separate power supply terminals are connected to the respective heat generators so that electric power may be applied independently to the respective heat generators. Heat generated from the inner and outer heat generators may thus be independently controlled.
Two-zone heaters include the following types. Japanese patent publication 2001-102157A discloses a heater having a ceramic substrate and two layers of heating elements embedded in the substrate. The calorific values in the inner zone and outer zone of each heating element are controlled so that a two-zone control system of inner and outer zones is realized.
It is desired to provide a design for adjusting the heat generation density from a heating resistance embedded in a ceramic heater, depending on the actual environmental conditions where the heater is set. For example, when a heating resistance (wound body) having a coil spring shape is embedded in a ceramic substrate, the heat generation density per unit area can be raised by increasing the winding number or winding diameter (coil diameter) or by reducing the wire diameter. Further, the heat generation density per unit area can be lowered by reducing the winding number or winding diameter (coil diameter) or by increasing the wire diameter.
Normally, the material of a heating resistance embedded in a ceramic substrate should be a high melting point metal which does not melt or easily deform at the sintering temperature of ceramics, and thus is limited. For example, when a wound body of molybdenum wire or tungsten wire is used, it is difficult to change the winding diameter or wire diameter in a single wound body due to the limitations of the manufacturing process. It is thus necessary that a plurality of wound bodies having the different winding diameters, winding numbers or wire diameters are joined and electrically connected with each other before the connected wound bodies are embedded in a ceramic substrate.
According to Japanese Patent publication 2003-272, 805A, for example, when a heating resistance composed of a tungsten coil is embedded in a ceramic substrate, two kinds of heating resistances having different wire diameters and winding diameters are used and are mechanically connected with each other using a spherical connecting terminal. It is thus possible to embed and combine two or more kinds of heating resistances having different wire diameters or winding diameters in a single ceramic substrate so that hot and cold spots on the surface of the substrate can be reduced.
The material of a heating resistance embedded in a ceramic substrate is, for example, a high melting point metal such as tungsten or molybdenum. A wire made of such high melting point metal is hard, brittle, hard to deform and is thus difficult to process. The terminal described above is thus necessary for connecting a plurality of heating resistances having the different wire diameters. However, if the resistance at the connecting part of the heating resistance and the terminal in use is raised, hot spots may be observed on the heating face. Moreover, for assuring excellent mechanical strength and reliability of the connecting parts of the heating resistance and the terminal, the structure, shape and method of connecting the connecting part need to be studied in detail. The manufacturing process required for producing the connecting part may become troublesome.